1. Field of the Invention
The present invention relates to a volume holographic memory and an optical information recording and reproducing apparatus using the volume holographic memory.
2. Description of the Related Art
Conventionally, a holographic memory system is known as a digital recording system using the principle of holography. The holographic memory system records and reproduces digital data to/from a memory medium of a photorefractive crystalline such as lithium niobate (LiNbO3). The photorefractive effect is a phenomenon in which electric charges generated by photoexcitatlon move in crystals to thereby form a spatial electric field distribution, which combines with a primary electro-optical effect (i.e., Pockels effect) to change a refractive index of the crystals.
With regard to a ferro-electric crystal which exhibits the photorefractive effect, a refractive index change normally responds to an optical input pattern of 1000 or more lines per 1 mm, and the effect generates in real-time at a response speed on the order of micro-second to second, depending upon a material used. Therefore, such crystal has been studied as a real time hologram medium, which does not need developing, with respect to various applications.
The holographic memory system can record and reproduce data in a two-dimensional plane page unit, and perform multiple recording with the use of a plurality of pages. The volume holographic memory enables three-dimensional recording with the memory medium being of three-dimensional configuration such as a rectangular parallelepiped. The volume holographic memory is a kind of Fourier transform holograms. Data is recorded in a dispersed manner by unitary image pages in a three-dimensional space of the memory. An overview of the holographic memory system will be described hereinbelow with reference to FIG. 1.
In FIG. 1, an encoder 25 converts digital data to be recorded in a volume holographic memory 1, into a dot pattern image of light and darkness in a plane, and rearranges the image in a data arrangement of, for example, 480 bits in a line and 640 bits in a row to generate a unitary-page sequence data. The unitary-page sequence data is supplied to an SLM (Spatial Light Modulator) 12 such as a panel of a transmission type Thin Film Transistor Liquid Crystal Display (hereinafter, referred to as “TFT-LCD” or “LCD”).
The SLM 12 performs a modulation processing by the processing unit of 480 pixels in line and 640 pixels in row, which corresponds to a unitary page. More particularly, the SLM 12 performs light modulation of a light beam or a source beam into an on/off signal of spatial light, corresponding to the unitary-page sequence data from the encoder 25. The modulated source beam or a signal light beam (hereinafter, referred to simply as “signal beam”) is conducted to a lens 13. More specifically, the SLM 12 passes therethrough the source beam in response to the Boolean value “1” of the unitary-page sequence data, which is an electric signal, and shuts off the source beam in response to the Boolean value “0” to thereby achieve electro-optical conversion in accordance with the contents of respective bits in the unitary page data. Accordingly, the signal beam of the unitary page sequence is generated by modulation of the source beam.
The signal beam is incident upon the volume holographic memory 1 through the lens 13. In addition to the signal beam, a reference light beam (hereinafter, referred to simply as “reference beam”) is incident upon the volume holographic memory 1 at an angle β (hereinafter, referred to as “incident angle β”) relative to a predetermined reference line perpendicular to an optical axis of the signal beam.
The signal beam and the reference beam interfere with each other within the volume holographic memory 1, and the resulting interference fringe is stored as a refractive index grid within the volume holographic memory 1, whereby recording of data is effected. Also, recording of three-dimensional data is made possible by angular-multiplexed recording of a plurality of two-dimensional plane data with variance of the incident angle β.
When reproducing the recorded data from the volume holographic memory 1, only the reference beam is made incident upon the volume holographic memory 1 at the same incident angle β as at the time of recording toward the center of a region where the signal beam and the reference beam intersect with each other. That is, reproducing the recorded data is different from recording data, in that the signal beam is not made to be incident. Therefore, diffracted light from the interference fringe recorded in the volume holographic memory 1 is conducted to a CCD (Charge Coupled Device) 22 in a light detector through a lens 21. The CCD 22 converts light and dark patterns of the incident beam into variations in intensity of an electric signal to output to a decoder 26 an analog electric signal having a level corresponding to brightness of the incident beam. The decoder 26 compares the analog signal with a predetermined amplitude (i.e., slice level) to reproduce data consisting of the corresponding “1” and “0”.
Because recording is performed in a two-dimensional plane data sequence within the volume holographic memory as described above, the incident angle β of the reference beam is varied to enable the angular multiplexed recording. That is, the incident angle β of the reference beam is varied to enable of defining a plurality of two-dimensional planes wherein the plane is a unit of recording, within the volume holographic memory. Therefore, three-dimensional recording can be achieved. An example of angular multiplexed recording is described in Japanese Unexamined Patent Publication Kokai Nos. 45-142979 and H10-97174.